The object of the invention is a lid structure for a food packaging container, which lid structure is of the type presented in the preamble of the attached claim 1.
There are several known lid structures used for closing food packaging containers, which lid structures are paper-based and comprise several layers of plastic on their lower surface. This kind of laminate structure is sealed fast to the edge flange of the packaging container that is intended as the actual storage space for the food. The lid must be, on the one hand, capable of being attached fast to the packaging container and, on the other hand, it must be easily detachable, in such a way that it can most preferably be pulled off completely from the packaging container, i.e. good peelability.
The lid structure, which is a paper-based laminate, has been presented, for example, in the international publication WO 95/10413. What is presented here is a gas barrier layer on the lower surface of the paper layer, attached to the paper by means of a sealing layer, which gas barrier layer may, for example, be of polyamide, and under it, a peelable layer that is heat-sealable to the packaging container, attached again to the gas barrier layer by means of a sealing layer.
Other paper-based laminates comprising various plastic layers, which laminates can be used as lid structures that are heat-sealable to the packaging container, are presented in the EP patent 258527, EP published application 322189 and U.S. Pat. 4469258.
Paper-based laminates are intended to replace packaging laminates comprising an aluminium film, which are troublesome in terms of recyclability. Recently more attention has been paid than before to the recyclability of packaging materials. The aim is, for example, to sort wastes, in which case plastic and paper, for example, should be separated from each other In packaging laminates, the various components are difficult to separate from each other. Therefore, efforts have been made to use compostable materials, i.e. biodegradable materials, where decomposition is caused by the action of micro-organisms starting in suitable conditions
Many biodegradable packaging materials are known, in which the aim is to achieve a suitable sandwich structure as regards biodegradability, on the one hand, and the gas and water vapour barrier properties required of a food packaging container on the other In order to meet these requirements, several biodegradable plastics have been developed which can be used in the plastic layers of paper-based packaging laminates. Until recently, the scope of application has been liquid packaging containers and flexible packaging papers used in bag-like packages. Until now it has not been possible to apply the above-mentioned idea to lids which are stripped off intact, i.e. which are peelable.
The aim of the invention is to eliminate the above-mentioned disadvantages and to provide a lid structure which, on the one hand, acts as a barrier according to the requirements for food products, is biodegradable and, in addition, can be stripped off intact from the packaging container, i.e. is peelable. In this connection peelability refers to the fact that the body material of the lid, i.e. paper, and, on the other hand, one or more plastic layers remain attached to the lid, while the lowest plastic layer may remain fast to the edge at the packaging container edges. To achieve this aim, the main characteristic of the lid structure relating to the invention is what has been presented in the characterising part of the attached claim 1.
The lid is a paper-based laminate made, for example, of paper or paperboard, under which there is one or more layers of biodegradable plastic, such as polylactide and/or polyhydroxy alkanoate, together with layers that may be of some other biodegradable plastic. The outermost, i.e. the lowest layer when the packaging container is in the position for use, which layer is attached to the packaging container, is a biodegradable copolymer (ester copolymer) containing ester bonds, to the ester parts of which copolymer a comonomer has been linked by ester or amide bonds. This outermost layer provides peelability.
Peeling occurs so that the peelable boundary layer is between the layer of biodegradable ester copolymer and the layer on its inner side, in which case, when the lid is removed, this outermost layer remains fast to the edge of the packaging container and the part of this layer which is in the middle of the packaging container comes off together with the lid.
Alternatively, the lower surface of the lid can be formed of material whose outer surface forms a peelable boundary layer, such as styrene lacquer.
Polylactides and polyhydroxy alkanoates, such as hydroxybutyrate/hydroxyvalerate copolymer are known biodegradable plastics. The thin outermost layer of ester copolymer is also biodegradable, which means that the entire lid is compostable. When styrene lacquer is used as the lower surface of the lid, the amount used is so small that it does not impair compostability.